Method of sensitizing zinc telluride

ABSTRACT

Zinc telluride can be sensitized by annealing in the presence of vapors of both zinc metal and a Group IIIA metal dopant. The sensitized zinc telluride exhibits photoconductivity.

United States Patent [191 Tan et al.

[ 1 June 28, 1974 5 METHOD OF SENSITIZING ZINC TELLURIDE [75] lnventors:Yen T. Tan; Raiinder P. Khosla;

John R. Fischer; Deepak K. Ranadive, all of Rochester, NY.

[73] Assignee: Eastman Kodak Company,

Rochester, NY.

[22] Filed: Mar. 13,1972 [2]] Appl. No.: 234,382

[52] US. Cl....f. 96/].5, 250/501, 423/508,

. I 7 423/509 [581 Int. Cl. 603g 5/02, G03g 5/04 [58] Field of Search252/501; 96/] R, I PC, 96/].5; 423/508, 509

[56] References Cited UNlTED STATES PATENTS 3: $0 w, li filfftfilz 9l3,104,229 8/!963 Koelmans et al. 252/50! 3,174,823 3/l965 Kopelman423/509 3,188,594 6/l965 Koller ct al. .252/50l X 3,462,323 8/1969Groves 423/509 X 3,551,763 12/1970 Hakki 423/508 X OTHER PUBLICATIONSPrimary Examiner-Roland E. Martin, Jr. Attorney, Agent, or Firm-DennisM. DeLeo 5 7] ABSTRACT Zinc telluride can be sensitized by annealing inthe presenee of vapors of both zinc metal and a Group IIIA metal dopant.The sensitiz ed zinc telhiride eiz hibits photoconductivity.

. 5 h me N. p iii fl i about 750 to about 900C.

FIELD OF INVENTION This invention relates to semiconductors and tomethods for preparation thereof. In particular, this invention relatesto means for preparing p-type semiconductors which exhibitphotoconductivity.

DESCRIPTION OF PRIOR ART Zinc telluride is a well known compound. Purezinc telluride is relatively electrically conductive and is notparticularly photoconductive. Various attempts have previously been madeto alter zinc telluride. For example, Bube and Lind in Phys. Rev. 105,1711, (1957), first observed photoconductivity in zinc telluridecrystals which are doped with indium and aluminum. Further work done byMakaranko and Rybalka reported in Soviet Physics, Semiconductors, 4,847, 1970) involved infrared quenching in zinc telluride which containediodine, indium or gallium impurities. Although some photoresponse wasnoticed in these works, the observed, pho'tocurrent was relativelysmall. Bube and Lind, for instance, reported a current of only 40microamps with an applied voltage of 100 volts and an intensity of 1.5mw/cm Although this prior-work is interesting, there still exists a needfor techniques to prepare zinc telluride crystals having relatively highphotoconductivity. I

SUMMARY OF THE INVENTION We have found that essentially pure zinctelluride can be rendered photoconductive by an annealing technique.This process results in a p-type'semiconductor having usefulphotoconductive properties. The resultant p-type materials are useful inelectrophotography, in semiconductor junction devices, inphotoconductive cells and the like. In addition, the present processprovides a means for varying the temperature for peak photosensitivityof the compound by varying the annealing time. 9

DESCRIPTION OF THE PREFERRED EMBODIMENTS The objects of the presentinvention are accomby the presence of zinc and the addition of electrondonors which increase the so-called compensation ratio.

As mentioned above, pure zinc telluride has a relatively highconductivity. This is believed to be the result of native acceptors nearthe valence band. These native acceptors, which are theorized as beingzinc vacancies, allow hole migration in the valence band giving rise toconductivity. In order to reduce this conductivity, we anneal in thepresence of zinc and a Group IIIA metal. The presence of the zinc vaporduring the annealing process tends to reduce the concentration of nativeacceptors which aids in the reduction of conductivity. In addition, weintroduce an impurity (that is, a Group IIIA metal dopant) within theband gap of the zinc telluride crystal. The introduction of this dopantinto the band gap results in compensation of the native acceptor by theionization of the donor dopant.

After suitable annealing, the zinc telluride is quenched to rapidly coolthe annealed compound to about room temperature. The material isquenched while the zinc telluride is maintained in the sealed containerwith the zinc vapor and Group IIIA metal vapor. This quenching is donein order to maintain the high temperature defect structure of thetreated crystal. A typical time for annealing prior to quenching wouldbe a minimum of about 10 hours. The material must be annealed for aperiod of time sufficiently long so that the zinc and Group IIIA dopantwill diffuse beneath the surface of the zinc telluride crystal. Forpractical purposes, 20 hours is a preferred minimum to insure sufficientdiffusion beyond the surface. The material can be annealed for as longas about 150 hours. Further annealing beyond this does not give rise toany significant change in properties. By varying the annealing time, onecan vary the temperature for peak photosensitivity. This allows one toessentially obtain a material having a peak photosensitivity at adifferent temperature.

The starting zinc telluride crystal should be pure material. It may beoff-stoichiometric, but should contain no more than about 0.3 molepercent excess of telluriuin. If a greater excess than this is present,the time plished by a process of annealing zinc telluride in thepresence of vapors of zinc and vapors of a dopant followed by cooling.The dopant used in this invention is a Group lIlA metal such as galliumand indium, etc.

7 In accordance with this'invention, the zinc telluride to be treated isplaced in a sealed container and annealed at elevated temperatures inthe presence of vapors of both zinc and a Group IIIA metal. Theannealing can be carried out at temperatures between about 600C and themelting point of the zinc telluride. The material can be treated attemperatures below about 600C, but the time required to produce usefulresults (e.g.-, to reduce the hole concentration sufficiently) becomesexcessive. At temperatures above about 950C, the material begins toevaporate and special precautions must be taken to prevent condensationon cooler portions of the container. From the standpoint of efficiency,the preferred annealing temperature range is The exact mechanism of thepresent process is not fully understood at this time. However, itappears to be a combined effect of a reduction of hole concentration ofannealing is changed significantly and difficulties arise in achieving aproduct useful as a photoconductor or the like. It appears that thepresence of a large excess of tellurium essentially negates the effectof the presence of the zinc vapors during annealing and poor results arethus obtained. Zinc telluride crystals useful fortreatment by thepresent invention can be prepared by standard techniques known in theart. The zinc telluride crystals can conveniently be prepared by theprocedures of J. Steininger'and R. E. England as described in Growth ofSingle Crystals of ZnTe and ZnTe Se, by Temperature Gradient SolutionZoning, Transactions of the Metallurgical Society of AIME, Volume 242,page 444 (1968). Evaporated layers of zinc telluride can also be treatedby the process of this invention.

The following embodiments are included for a fur driven through atemperature gradient of 25c/in. The crystals thus formed contain about0.12 percent excess tellurium. The bulk crystals are cut into Hallspiders about 1mm thick and weighing about 0.1 to about 0.3 grams. Theresultant Hall spiders are placed in a quartz vacuum ampoule about 5 tocc. in size with about 0.2 grams of zinc metal of the purity used tomake the original crystal and about 0.1 gram of similarly pure galliummetal. The ampoule is evacuated to about 1 X 10" torr and sealed. Theampoule is then annealed at about 835C for about 39 hours and quicklyquenched to room temperature in an oil bath. The annealed sam ple isthen removed and etched in concentrated solution of hot sodiumhydroxide. Lithium contacts are applied by placing a small drop of 10molar LlNOg solution on the contact area and then heating the sample ina flowing hydrogen atmosphere at about 350C for a minute. This techniqueis described by Aven and Garwacki in J. Electrochem. Soc, 14, 1063(1967). Electroless gold from an l-lAuCh solution is deposited on thelithium contacts. The resultant lithium-gold electrodes give good, lowresistance ohmic contacts over a range of temperatures. The darkresistance of the sample is measured and found to be greater than 10ohms. The photoresponse is measured by determining the specificsensitivity S of the sample. Specific sensitivity, as described by R. H.Bube, RCA Eng., 5, 28 (1960), is defined as S AiP/VP where A i is thechange in photocurrent, 1 is the distance between electrodes, V is theapplied voltage and P is the light absorbed in watts. The specificsensitivity is independent of the applied field and the intensity of theillumination if both vary linearly with photocurrent. The applied fieldis varied between 2 to v/cm. The specific sensitivity is calculated forsamples illuminated with a tungsten source and a monochrometer at 560nm. The intensity ofillumination is about 1 mw/cm The result of thesemeasurements indicate the specific sensitivity at room temperature to be0.02. The peak specific sensitivity at 200K is 0.7, which compares withthe peak specific sensitivity of 0.1 to 2.0 obtained with highlysensitized cadmium sulfide. A bulk sensitivity may be calculated for thenon-current-carrying electrodes of the Hall spider in order to excludecontact resistance. The bulk sensitivity 5,, is defined by the followingequation: S A0'(Al l )l' /p where A0- is the bulk photoconductivity andA is the cross-sectional area. The bulk sensitivity of the sample is 1.6at about 200K and 560 nm. Embodiment 2 Embodiment l is repeated with theexception that the sample is annealed for 64 hours and the peak specificsensitivity is 0.16 and the bulk sensitivity is 2.9. Embodiment 3Embodiment l is repeated except that the sample is annealed for 99 hourswith the resultant peak bulk sensitivity of 2.2 at room temperature andabout 560 nm.

' The dark resistance is 10 ohms.

Control No. l

Embodiment 1 is repeated with the exception that the ampoule containsonly zinc telluride and gallium metal. The ampoule is annealed for 64hours and quenched as previously. There is practically nophotoconductivity at room temperature of the resultant sample and thepeak specific sensitivity is 10" at about l43K and about 560 nm. ControlNo. 2

Embodiment l is repeated once more except that the ampoule contains onlyzinc telluride and zinc metal with annealing for about 50 hours. Thespecific sensitivity at K is about 10 for the resultant material and thedark resistance is about 300 ohms.

Control No. 3

Embodiment l is repeated except that the sample is not subjected toannealing. There is no noticeable photosensitivity and the ratio oflight to dark conductivities (signal to noise ratio) is less than 1.01.The dark resistance is about ohms.

Embodiment 4 Embodiment 1 is repeated with the exception that theampoule contains zinc telluride, zinc metal and indium metal and isannealed for about 63 hours at 850C. The resultant sample shows a peakspecific sensitivity of 0.08 and a bulk sensitivity of 2.3 at K and 560nm.

Control No. 4

Embodiment 4 is repeated with the exception that the ampoule containszinc telluride and indium metal only and is annealed for 62 hours at835C. The resultant sample shows practically no photosensitivity at roomtemperature and a peak specific sensitivity of about 10' at 154K and 560nm.

Embodiment 5 The annealed zinc telluride formed by the procedure ofEmbodiment l is ground into a powder and combined with an electricallyinsulating polymeric binder and coated onto a conductive support. Theresultant electrophotographic element is then charged under a coronacharger and subjected to illumination. The charge applied to the elementis dissipated in the areas of exposure in a manner proportional to theamount of illumination received.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

We claim:

1. A method of preparing photoconductive zinc telluride consistingessentially of the steps of:

1. annealing zinc telluride crystals, having no greater than about 0.3mole precent excess tellurium, said annealing occurring a. at atemperature in the range of about 600C to the melting point of said zinctelluride, b. for at least about 10 hours and c. in the presence ofvapors of zinc and a Group [Ila metal dopant, wherein said vapors ofzinc and Group Illa metal dopant are present at the saturation vaporpressure for the temperature of said annealing; and

2. quenching to cool said annealed zinc telluride.

2. The method as described in claim 1 wherein said annealing occurs at atemperature in the range of about 750C to about 900C.

3. The process as described in claim 1 wherein said dopant is selectedfrom the group consisting of gallium and indium.

4. A photoconductive zinc telluride compound consisting essentially ofzinc telluride which has been annealed in the presence of a gaseousGroup 111A metal dopant by the process of claim 1.

5. The method as described in claim 1 wherein said resultant annealedzinc telluride has a peak specific sensitivity of greater than 10*.

2. quenching to cool said annealed zinc telluride.
 2. The method asdescribed in claim 1 wherein said annealing occurs at a temperature inthe range of about 750*C to about 900*C.
 3. The process as described inclaim 1 wherein said dopant is selected from the group consisting ofgallium and indium.
 4. A photoconductive zinc telluride compoundconsisting essentially of zinc telluride which has been annealed in thepresence of a gaseous Group IIIA metal dopant by the process of claim 1.5. The method as described in claim 1 wherein said resultant annealedzinc telluride has a peak specific sensitivity of greater than 10 3.